This invention relates to catalysts composed of a zeolite of the faujasite type which has been exchanged to reduce the sodium content of the zeolite and has been combined with a matrix containing clay and alumina. Such catalysts are widely used in commercial catalytic cracking and are composed of a Y zeolite which has been exchanged with a salt solution, such as ammonium, rare earth or both ammonium and rare earth salt solutions.
It has been reported that they are formed by spray drying a water slurry of the exchanged zeolite, alumina and clay to which additives such as ammonium polysilicate (see Lim, et al, U.S. Pat. No. 4,086,188) may be added to improve the attrition resistance of the catalyst.
Many forms of inorganic oxide gels, including alumina hydrate gels, have been suggested in the prior art as or as part of the matrix. The literature and patents disclosing such composition is voluminous. The combination of a matrix formed of clay and crystalline alumina hydrate of the pseudoboehmite form are disclosed in Secor et al U.S. Pat. No. 4,010,116 and Lim et al U.S. Pat. No. 4,086,188, and Lim et al, Ser. No. 3,407, now U.S. Pat. No. 4,206,085.
Such catalysts are rated for their activity by a bench scale test. Such a test determines the percent conversion of a feed stock in a cracking process employing a catalyst which has been steamed for two (2) hours at 1450.degree. F. The percent conversion attained using such a steamed catalyst is known as M activity. The activity determined using a catalyst which has been steamed at 1550.degree. F. for two (2) hours is termed S+ activity.
The M activity represents the so-called fresh activity, that is the activity when the catalyst is first introduced into the cracking system and the S+ activity represents the activity of the recirculated catalyst which has passed through the conversion and regeneration cycle, wherein it is subjected to a high temperature condition in the regenerator, and is then recirculated to the reactor.
The effect of the high temperature hydrothermal conditions in the regenerator results in a partial deactivation of the catalyst. The S+ activity is thus a measure of the stability of the catalytic properties of the catalyst as measured by its activity.
The pseudoboehmites differ in their rheological properties. The Type A pseudoboehmite and B Type pseudoboehmite are identified by a rheological test disclosed in Lim et al, U.S. Pat. No. 4,206,085. Both A Type pseudoboehmite and B Type pseudoboehmite will provide catalysts of the above Type compositions of satisfactory S+ activity.
As appears from the disclosure in the Lim, et al U.S. Pat. No. 4,206,085, in catalysts of the above composition, when tested by the bench scale test described in the patent, the M activity is largely insensitive to the concentration of pseudoboehmite and for this purpose pseudoboehmite of the B types are substantially as effective as those of the A type.
The magnitude of the S+ activity of a given composition of an exchanged zeolite and a matrix of clay and pseudoboehmite increases as the concentration of the pseudoboehmite is increased, while the concentration of the zeolite remains substantially the same.
The magnitude of the S+ activity as appears from the above is dependent on the character of the cation population, the silica to alumina molar ratio of the faujasite type zeolite as well as the concentration of the zeolite, clay and pseudoboehmite.
The attrition resistance is measured by a bench scale test described in the Secor et al Patent. The test measures the loss due to attrition after five (5) hours of the test as a percent of the original sample weight. This percentage figure is termed the Attrition Index. The lower this figure, the better the attrition resistance. Acceptable Attrition Index, for example, is one less than thirty (30), i.e., 30% loss after five (5) hours, and preferably twenty (20) to twenty-five (25) or less. A very desirable Attrition Index is less than twenty (20).
Not all forms of pseudoboehmite are equally suitable to impart a desirable attrition resistance to the spray dried catalyst.
As set for in said Lim et al supra, U.S. Pat. No. 4,206,085, the Type A alumina is superior to the Type B alumina in imparting a desirable attrition resistance. The pseudoboehmites of the A Type are materially improved as an addition to the matrix of a zeolite catalyst by the inclusion of ammonium polysilicates or of a silica gel of sufficiently fine particle size. (See Lim et al, U.S. Pat. Nos. 4,086,188 and 4,206,085.)
It has also been shown in Lim et al U.S. Pat. No. 4,206,185 that the unmilled B Type may not be employed as a total substitute for the A Type without substantially decreased attrition resistance. Only a minor fraction of the A pseudoboehmite may be substituted by the B Type if the B Type be sufficiently milled.
The said Lim et al patents (U.S. Pat. Nos. 4,086,188 and 4,206,085) are incorporated herein by this reference.